1. Field of the Invention
The present invention relates to a light emitting device, more particularly to an organic electroluminescent device having an improved display quality.
2. Description of the Related Art
Organic electroluminescence is a phenomenon wherein excitons are formed in an organic (low molecular or high molecular) material thin film by re-combining holes injected through an anode with electrons injected through a cathode, and a light of specific wavelength is generated by energy from thus formed excitons.
The basic structure of an organic electroluminescent device includes a transparent substrate, a plurality of anode electrode layers and a plurality of cathode electrode layers, disposed on the glass substrate so as to overlie each other, and an organic material layer interposed between the two electrode layers, wherein applying a voltage to the organic material layer through the two electrode layers allows the injected electrons and holes to re-combine each other and create an electroluminescent light.
FIG. 1A is a block diagram illustrating an organic electroluminescent device.
Referring to FIG. 1A, the organic electroluminescent device comprises a panel 100 and a driver 102 electrically connected thereto.
The panel 100 comprises a plurality of pixels E11 to E55, which correspond to luminescent areas that are defined as overlying areas of a plurality of anode electrode layers (hereinafter, referred to as “anode lines”) A1 to A5 and a plurality of cathode electrode layers (hereinafter, referred to as “cathode lines”) C1 to C5.
The driver 102 comprises a controller 104, a first scan driving circuit 106, a second scan driving circuit 108 and a data driving circuit 110.
The anode lines A1 to A5 are electrically connected to a data driving circuit 110 outside the panel 100 through data lines D1 to D5 to which the anode lines A1 to A5 are coupled, while the cathode lines C1 to C5 are electrically connected to scan driving circuits 106 and 108 outside the panel 100 through the scan lines S1 to S5 to which the cathode lines C1 to C5 are coupled.
The first scan driving circuit 106 is electrically connected to the scan lines S1, S3 and S5 extended in a first direction to transmit first scan signals to the cathode lines C1, C2 and C5 through the corresponding scan lines S1, S3 and S5. The second scan driving circuit 108 is electrically connected to the scan lines S2 and S4 extended in a second direction, which is different from the first direction, to transmit second scan signals to the cathode lines C2 and C4 through the corresponding scan lines S2 and S4.
A controller 104 transmits a first control signal CS1 to the first scan driving circuit 106, a second control signal CS2 to the second scan driving circuit 108, and a third control signal to the data driving circuit 110 to control the operations of the driving circuits 106, 108 and 110.
The data driving circuit 110 provides a data current corresponding to a display data input from the outside to the anode lines A1 to A5 through the data lines D1 to D5.
FIG. 1B is an equivalent circuit diagram of the panel 100 of FIG. 1A, illustrating an aspect of the cathode lines C1 to C2 being connected to the scan driving circuit (106 and 108 of FIG. 1A, indicated as a ground and a scan voltage V1 herein). In addition, FIG. 1C is an equivalent circuit diagram of some pixels of FIG. 1A, and FIG. 1D is a timing diagram illustrating a scan voltage and a data current provided through a scan line and a data line respectively.
Referring to FIG. 1B, some cathode lines C1, C3 and C5 of the cathode lines C1 to C5 are connected to scan lines S1, S3 and S5, which are extended in a first direction from one ends of the cathode lines C1, C3 and C5 to be connected to a scan voltage V1 or a ground, while the other cathode lines C2 and C4 are connected to scan lines S2 and S4, which are extended in a second direction from one ends of the cathode lines C2 and C4 to be connected to the scan voltage V1 or the ground.
Hereinafter, the operation of the pixels E11 to E55 will be described. Only, for convenience of the explanation, as shown in FIG. 1B, it is assumed that the resistance of each scan line S1 to S5 is 60Ω, and the resistance of each cathode line C1 to C5 of between the pixels E11 to E55 is 10Ω.
First, the first scan line S1 is connected to a ground while the other scan lines S2 to S5 are connected to the scan voltage V1, which has the same level as a driving voltage to drive the pixels E11 to E55. Here, only the pixels on the cathode line C1 connected to the scan line S1 emits a light because any pixel E11 to E55 emits a light only when the scan line S1 to S5 connected to its corresponding cathode line C1 to C5, is connected to the ground.
Next, the second scan line S2, which is extended in a different direction as that of the first scan line S1, is connected to the ground, while the other scan lines S1, S3, S4 and S5 are connected to the scan voltage V1. As a result, the pixels E12 to E52 on the cathode line C2, connected to the second scan line S2, emit a light.
For the foregoing case, line resistance components R11 to R51 of the pixels E11 to E51 on the cathode line C1 and line resistance components R12 to R52 of the pixels E12 to E52 on the cathode line C2 will be compared with reference to FIG. 1C.
Referring to FIG. 1C, the resistance components R11 and R12 of the adjoining two pixels E11 and E12 on the anode line A1 have a resistance difference of 40Ω, the resistance components R21 and R22 of the adjoining two pixels E21 and E22 on the anode line A2 have a resistance difference of 20Ω, and the resistance components R31 and R32 of the adjoining two pixels E31 and E32 on the anode line A3 have the same resistance as each other. Furthermore, the resistance components R41 and R42 of the adjoining two pixels E41 and E42 on the anode line A4 have a resistance difference of 20Ω, and the resistance components R51 and R52 of the adjoining two pixels E51 and E52 on the anode line A5 have a resistance difference of 40Ω.
Hereinafter, the influence of these line resistance differences on the brightness of each pixel E11 to E55 will be described with reference to FIG. 1D. Only, the case of the pixel E11 emitting a light will be provided as an example.
Referring to FIG. 1D, a data current I1 is provided to the pixel E11 through the data line D1 when the scan line S1 is at the low logic state. In theory, the data current I1 has a predetermined value Iw while the scan line S1 is at the low logic state, but in reality, the data current I1 has a lower value Iu than the predetermined value Iw as shown in FIG. 1D. That is, a data current is influenced by its corresponding resistance, and thus the brightness of the pixels E11 to E55 may have a variance due to the resistance components R11 to R55.
In the foregoing example, the case that the brightness of the pixels E11 to E55 is lowered due to the resistance components R11 to R55 has been provided, but the brightness of the pixels E11 to E55 may be increased in another case in another example.
Hereinafter, the operation of the panel 100 will be described in detail.
Referring again to FIG. 1C, the resistance components R11 and R12 of the pixels E11 and E12 on the anode line (A1 of FIG. 1B) have a greater resistance difference, therefore a considerable brightness difference between the pixels E11 and E12 may occur due to the resistance components R11 to R12 even though the same data current is provided to the pixels E11 and E12.
In addition, the brightness difference may occur between the pixels E12 to E55 on the other anode lines A2 to A5. But the brightness difference is conspicuous between the pixels E11 to E15 and E15 to E55 on the anode line (A1 and A5 of FIG. 1B) disposed at the edge of the panel. As a result, the brightness difference is repeated along the pixels E11 to E15 and E15 to E55 on the anode lines A1 and A5, thereby creating stripes, i.e. “pectination.” Usually, the pectination generates along the left and right edges of the panel 100 to be noticeable to the users.
For the foregoing reasons, there is a need for a flat panel display device, such as a light emitting device, electroluminescent device or organic electroluminescent device, having an improved display quality without pectination.